During the production of electricity, generators are used that produce an alternating voltage around a certain frequency. The user of the produced electricity is aware of which frequency is involved and which voltage is provided on the electricity network. The user of the electricity network requires as clean electricity as possible. That is, the user requires electricity that is well-defined at the specified frequency and the specified voltage. In normal electricity networks that supply towns and factories, it is, however, normal for the electricity network to comprise low-frequency voltage distortions. Voltage distortions arise when various devices are cyclically connected into the network, which devices comprise, for example, capacitances and inductances.
Large power-hungry installations such as induction furnaces, compressors, lift motors, pumps, etc., act to increase the amount of voltage distortion in the electricity network. These loads often cause low-frequency (<25 Hz) and periodic fluctuations in the effective value of the voltage. The phenomenon is usually called flicker. The most noticeable effect of flicker is the annoyance that can be experienced when the light intensity from incandescent lamps fluctuates in line with the voltage variations caused by the flicker. Studies have shown that people are particularly sensitive to light fluctuations with a repetition frequency in the range 0.5 Hz to 25 Hz. At the level of maximum sensitivity (approx. 9 Hz), the relative voltage change needs only to be 0.25% for people to be able to perceive that the light from the incandescent lamp flickers. Problems with flicker arise primarily in areas with very heavy industry (ironworks and paper-mills, etc.) but can also arise in areas with weak electricity networks and in the vicinity of wind-power installations.
In the event of a dirty electricity network comprising flicker, it is of interest to know where the flickering source is located. An electricity producer wants to show that it is a consumer that makes the electricity network dirty and can, in such a case, demand that the consumer pay a penalty charge or corrects the problem to eliminate the flicker. A consumer, on the other hand, wants of course to show that it is not the consumer causing the problem. In addition, the consumer wants to have value for money and thus wants to show that it is the electricity producer that is supplying dirty electricity. Correcting the problem of flicker is often an expensive operation where all or parts of the network can need to be reconstructed in such a way that the internal impedance of the electricity network is reduced, for example by use of new and larger cables. Another way to correct the problem is to install counter measures against the flicker in question. Such counter measures are usually very expensive. An example of a counter measure is “Static Var Compensation (SVC)” which dynamically controls changes in the system.
Methods exist for determining the content of voltage flicker, which are described in the standard IEC-61000-4-15. Measuring instruments that record voltage flicker according to this standard show the presence of voltage flicker by calculating and displaying the parameters If1, Pst and P1t. There is, however, nothing in the standard or in existing measuring instruments that shows the direction to the flickering source in relation to the measurement point.
There is thus a great need for a method and an arrangement that can determine the direction of propagation of flicker. The direction of propagation of the flicker shows whether the flickering source is above or below a measurement point and can thus be of great use when the flickering source is to be traced.